Both sets of candidates for Indonesia's presidential race marked the official start of campaigning yesterday with prayers for safety and victory, and pep talks to team members, before setting out to win over 186 million eligible voters.

Over the next month, Jakarta Governor Joko Widodo and his running mate Jusuf Kalla, and former general Prabowo Subianto and his running mate Hatta Rajasa, will travel through the 5,248km-long archipelago in what many expect will be an intense contest to be Indonesia's seventh president.

This is the first time that residents of South-east Asia's largest economy will see a straight race for the presidential election. But observers feel the fact that voting for the July 9 polls will take place during the fasting month of Ramadan, which begins on June 28 - as well as during the World Cup, which kicks off next week - will lower temperatures somewhat.

The latest opinion polls show Joko, commonly known as Jokowi, still in the lead over Prabowo, but the gap between the two has narrowed in recent months.

A national survey by pollster Populi Centre done last week and released yesterday found 47.5 per cent of respondents would pick the Jokowi-Kalla ticket against 36.9 per cent for the Prabowo-Hatta ticket if the election were held now, with 14.4 per cent undecided and 1.2 per cent not choosing either.

"The ability of both pairs to draw sympathy and support from these undecided voters will be key, and both will have to make the most of the campaign period," Populi Center head Nico Harjanto said, noting that Jokowi supporters like his closeness to people, while Prabowo supporters appreciate his tough leadership.

Yesterday, Joko and leaders of his Indonesian Democratic Party-Struggle (PDI-P) attended a ceremony at the party's headquarters.

There, party chairman Megawati Sukarnoputri cut a traditional rice cone used to mark important occasions.

In the evening, both men presented their economic programmes to several hundred business leaders at a fund-raising event at the Ritz-Carlton Hotel.

Late at night, Joko and Megawati flew to Papua to meet residents today, while Kalla will be in Aceh.

"This is a form of concern for the people of Indonesia from end to end," Joko told reporters.

Prabowo, the patron of the Greater Indonesia Movement (Gerindra) party, began his campaign in Bandung, the capital of battleground province West Java, where he met leaders of political parties backing his campaign.

Accompanied by Golkar chairman Aburizal Bakrie, he had coffee and satay with street buskers, and the duo are set to meet various groups of supporters in the city today.

Hatta, meanwhile, attended a gathering of members of Muslim group Nahdlatul Wathan and will campaign outside the capital today.

Yesterday also saw the first defamation complaint. A group of Prabowo supporters lodged a police report against former intelligence chief A.M. Hendropriyono, a Jokowi campaign adviser, after he was reported as describing Prabowo as a "psychopath" at a panel discussion on Tuesday.

Criminal defamation is an offence under Indonesian law, and complainant Alfons Loemau said Mr Hendropriyono had smeared Prabowo's reputation, the website of main newspaper Kompas reported.

There will also be a series of five televised campaign debates, and both pairs of candidates will take to the stage at the first session next Monday, on the theme of democracy, governance and the rule of law.

The next session, between Joko and Prabowo, will take place a week later.